Electronic pressure sensitive transducer apparatus

ABSTRACT

A pressure responsive, variable resistance, analog switch has first and second conductors interleaved in spaced-apart relationship and disposed on a base member. An insulative spacer ring is positioned around and rises above the first and second conductors. A resilient cover sheet is attached to the top of the insulative spacer ring in spaced relationship over the conductors to define an enclosure between the resilient cover sheet and the base member. A pressure sensitive resistive conductor composition is disposed on the resilient cover sheet or on the conductors in the enclosure to interconnect a resistance between the first and second conductors when the resilient cover sheet is depressed against the conductors. The amount of resistance so interconnected varies inversely to the amount of pressure exerted.

BACKGROUND OF THE INVENTION

The present invention relates to pressure sensitive variable resistancedevices and in particular relates to pressure sensitive variableresistance switches particularly useful on a keyboard for an electronicmusical instrument which actuates the generation or changing of a toneand thereafter causes analog variations in the volume or tonalcharacteristics in response to the application of a greater or lesserdepression of force on the switch.

The generation of musical sounds by electronic means is well known.However, one problem which exists in most electronic instruments is theinability to continuously vary either the volume or the tonal quality ofthe sound generated. This inability limits the musician's freedom ofmusical expression. The present invention provides a novel yet simplepressure responsive analog switch having a contact resistance whichvaries inversely to the amount of pressure applied to depress the analogswitch. When used in electronic musical instruments, a plurality of suchanalog switches may be placed side by side in an elongated fashion toprovide a keyboard or one such switch may be used to effect changes intone by altering the characteristics of one or more tone generatingcircuits in the musical instrument.

Pressure sensitive analog switches have been known. For example, both inRuben, U.S. Pat. No. 2,375,178, and Costanzo, U.S. Pat. No. 3,386,067,analog switches are disclosed which sandwich a fibrous or sponge-likelayer containing a conductive material between two conductor plates. Asthe two conductor plates are compressed together the number ofelectrically conductive paths through the sandwiched layer volumeincreases, thus decreasing the electrical resistance through that layer.In each of these devices, however, the resistive sandwich layer must beresilient to force the electrodes apart and disconnect most of theconductive paths when the compression force is released. Furthermore,the semiconducting sandwiched layer depends on macroscopic compaction toincrease the number of electrical conductive paths between the upper andlower conductor plates. Consequently, the sandwiched layer must have arelatively large thickness. Finally, in such devices the resiliency ofthe fibrous or sponge-like layer can decrease with use, thus causing adegeneration in the operating characteristics of the switch.

In Mitchell, U.S. Pat. No. 3,806,471, a pressure responsivesemiconductor material such as molybdenum disulfide was disclosed,placed between conductor plates to provide an adjustable resistor ortransducer. However, Mitchell relies on volume resistance, that is, theresistance through a relatively thick volume of the molybdenum disulfidelayer. The present invention on the other hand uses the contact orsurface resistance of a very thin layer of molybdenum disulfide. Morespecifically, Mitchell discloses a molybdenum disulfide volume(thickness) of 0.001 to 1.0 inch using molybdenum disulfide particles inthe range of 50 to 600 mesh to provide a high but finite number ofthree-dimensionally distributed current flow paths through the resistivematerial. Under compression, the number of current flow paths betweenthe particles in the volume increases, thus causing the resistance todecrease. The semiconductor volume layer is then permanently positionedand attached between two conducting electrodes.

By contrast, the present invention is exemplified by the use of particlesizes on the order of one micron and layer thickness, preferably lessthan 0.001 inch. Furthermore, since the variable resistance occursbecause of a greater or lesser number of surface contact locations, onesurface of the semiconductor layer must be at least initially spacedapart from one of the conducting electrodes. Depression of thespaced-apart conducting electrode against the surface of the thinsemiconductor layer results in a plurality of contact points being madealong the surface. These contact points increase as pressure is applied,thus decreasing the resistance between the conducting plates or contactson either side of the semiconductor layer. Of course, the surfacecontact semiconductor layer may be made of any suitable semiconductormaterial.

A significant advantage of the thin semiconductor layer of the presentinvention is that the semiconductor material used to form the layer maybe combined with a binder and a binder thinner and thereafter sprayed orsilk-screened onto the desired surface to form a layer having athickness as little as one mil or less. Manufacturing costs for bothlabor and materials are thus greatly decreased.

In Pearlman, et al., U.S. Pat. No. 4,044,642, a touch sensitiveresistance device is disclosed for use in musical instruments. However,the device uses a semiconductor material sandwiched between twoconductor plates in a manner similar to Ruben and Costanzo.Specifically, Pearlman, et al., uses a resilient material such as foamrubber or foamed synthetic polymeric material which has a particulatematerial such as graphite dispersed throughout. The switch structure hasa foam semiconductor layer and an insulator layer with an orificetherethrough sandwiched between two conductor plates. Thus, when acompression force is applied, the graphite-saturated resilient foamlayer deforms into the orifice in the insulator material to initiallymake electrical contact to thereby switch the musical instrument on.Thereafter, additional compression force causes the resistance betweenthe two conductor plates to decrease in the manner previously described,thereby altering the volume or tonal quality produced.

Because Pearlman, et al., uses a porous foam material there is noproblem of air compression in the cavity when the switch is depressedsince the air may easily escape and return through the porous resistivematerial. Furthermore, Pearlman, et al., depends on the physicalresiliency of the graphite-impregnated foam material, thus requiring asemiconductor layer of substantially greater thickness than with thepresent invention. In addition, a degradation in mechanical resiliencyof the semiconductor layer also causes a degeneration in switchperformance.

It is therefore desirable to provide an analog switch which has apressure sensitive variable resistance in the ON state but which doesnot rely upon the resiliency of the semiconductor layer to cause theswitch to turn to an OFF state when the compression force is removed.Furthermore, it is desired to provide an analog switch without relyingon the volume resistance through a relatively thick semiconductor layerpermanently attached between two conductive plates or electrodes.

SUMMARY OF THE INVENTION

The present invention comprises a pressure responsive analog switchhaving a resistance which varies inversely to the amount of compressionforce applied to the switch. Specifically, the analog switch has a basemember on which first and second spaced contact conductors are disposed.An insulative spacer is positioned on the base member around the contactconductors with a cover fixed to the insulative spacer, spaced above thecontact conductors. The space between the cover and the contactconductors defines an enclosure surrounded on its sides by the spacer.The cover is resiliently movable toward the contact conductors inresponse to an external compression force. A pressure sensitivesemiconductor ply is then positioned in the enclosure between the coverand the contact conductors for providing a variable resistance pathbetween the first contact conductor and the second contact conductorwhen the cover is moved into physical contact with them. The resistanceof the pressure sensitive semiconductor ply varies in response tovariations in the externally applied compression force. Finally, apassageway is provided between the enclosure and the external region ofthe analog switch for allowing free airflow into and out of theenclosure when the cover moves away from or towards the contactconductors.

In one embodiment, the pressure sensitive semiconductor ply comprises athin, pressure sensitive, semiconductor composition layer disposed onthe surface of the resiliently movable cover. In a second embodiment,the pressure sensitive semiconductor ply comprises a third conductor,such as a layer of silver, on the surface of the cover in the enclosureand a pressure sensitive semiconductor composition layer disposed on atleast one of the first and second contact conductors.

BRIEF DESCRIPTION OF THE DRAWINGS

A complete understanding of the present invention and of the aboveadvantages may be gained from a consideration of the followingdescription of the preferred embodiments taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a cross-sectional plan view of one embodiment of a pressuresensitive analog switch with the pressure responsive coating positionedbetween two conductor plates in a spaced relationship.

FIG. 2 is a cross-sectional plan view of a preferred embodiment of apressure responsive analog switch in accordance with the presentinvention.

FIG. 3 is a cross-sectional plan view of an alternative embodiment of apressure responsive analog switch with the thin resistive coating on theconductors.

FIG. 4 is a schematic representation of a pressure responsive analogswitch with the cover removed shown interconnected to a utilizationcircuit.

DETAILED DESCRIPTION

Referring first to FIG. 1, an analog switch in accordance with thepresent invention is shown comprising a first conductor plate 50 spacedfrom a second conductor plate 52 by spacers 54 to define a gap orchamber 60 between the first and second conductor plates 50 and 52. Atleast one of the conductor plates 50 or 52 is resilient so that it maybe depressed against the other conductor plate to close the switch.

The conductor plate 50 may comprise a flexible support sheet 64, such asMylar, with a thin conductive layer 66 of silver or other conductivematerial sprayed, screened or otherwise applied on the surface of thesupport sheet 64 adjacent the second conductor plate 52. The secondconductor plate 52 may comprise a rigid plastic base member 68 with athin copper surface 70 disposed thereon. Of course, it will beappreciated that the base member 68 may be flexible and the thin surface70 may be made of silver or other suitable conductive material. A lead56 and a lead 58 may be coupled to the silver layer 66 and the coppersurface 70 respectively to allow for electrical coupling of the analogswitch to a utilization circuit.

Finally, a thin semiconductor layer 62 of semiconductor material issprayed, screened or otherwise evenly applied on the copper surface 70.Alternatively, the semiconductor material 62 may be sprayed, screened orotherwise evenly applied on the conductive layer 66 or on both thecopper surface 70 and the conductive layer 66. The semiconductormaterial may be any suitable composition which is sprayable, screenable,or otherwise of a consistency which may be evenly applied to form asmooth exposed surface. For example, the semiconductor material may bemolybdenum disulfide particulate having particle sizes on the order ofone to ten microns mixed with a binder material such as resin to form aliquid. A resin thinner may be added to give the composition aconsistency suitable for spraying. The thin semiconductor layer 62 ofthe semiconductor material is then sprayed or screened on the conductivelayer 66 of the support sheet 64 or on the copper surface 70 on therigid base member 68. It will be appreciated, of course, that thesemiconductor layer may be of any thickness so long as there is anexposed smooth semiconductor surface. However, in order to conserve onsemiconductor material and to minimize surface irregularities which mayoccur when thick semiconductor layers are utilized, a thickness on theorder of about 0.001 inch or less is preferred.

The use of a very thin layer of sprayed or screened semiconductormaterial allows the semiconductor material to be resiliently moved bythe depression of the conductor plate 50. Furthermore, since it is asurface contact resistance effect and not a volume resistance thatcauses a decrease in resistance when pressure is applied, much lesssemiconductor material is required to be used and fabrication of theswitch is much faster, easier and less expensive than with prior artdevices. The minimum resistance through the semiconductor layer may beselected by control of the ratio of semiconductor material to binder.

Of course, it will be appreciated that the semiconductor material may bebrushed or screened or disposed on the selected surface in any suitableway so that a uniform, smooth exposed semiconductor surface is provided.It will also be appreciated that any semiconductor material may be usedso long as a large number of contact points are provided on thesemiconductor surface whereby variations in the pressure applied topress a second conductor against the semiconductor surface will causevariations in the number of contact points and hence, variations in theresistance across the semiconductor material. The resistance through thesemiconductor layer can be varied by varying the semiconductor materialto resin ratio. In the preferred embodiment, because the phenomenon isbased on surface resistance, not volume resistance, the weight ratio ofbinder to semiconductor material is about one to one.

Referring to FIG. 2, another embodiment of a pressure responsive,variable contact resistance analog switch 10 is illustrated having abase member 12 which may be rigid plastic, flexible Mylar (polyethyleneterephthalate) or any other suitable material. Contact conductors 13comprising spaced first and second contact conductors 14 and 16 aredisposed on one surface of the base member 12. An insulative spacermember 18 is affixed to the base member 12 around the contact conductors13. A cover 19 is then positioned on top of the insulative spacer 18thereby defining an enclosure or chamber 24 between the cover 19 and thecontact conductors 13.

In one embodiment, the cover 19 comprises a flexible support member 20which may, for example, be a thin sheet of Mylar. The side of theflexible support member 20 facing the contact conductors 13 is sprayedwith a pressure sensitive semiconductor composition layer 22 which may,for example, be a mixture of any suitable resin, e.g., acrylic resin,such as R-20 sold by Specialty Coatings & Chemicals, Inc. of NorthHollywood, Calif., and molybdenum disulfide. In one embodiment theliquid composition to be sprayed is made by mixing 5 to 10 millilitersresin, 40 milliliters resin thinner, and 8.5 grams of molybdenumdisulfide. Of course, it will be appreciated that numerous other resinand semiconductor material compositions may be used without departingfrom the spirit of the present invention. Specifically, materials suchas sponge iron powder and iron oxide, tungsten carbide powder, tin oxidepowder, boron powder or any other semiconductor material may be used,although molybdenum disulfide is preferred because of its low-noiselubricating characteristics.

The resultant cover 19 is glued or otherwise mechanically affixed to atleast portions of the top of the insulative spacer 18 so that thepressure sensitive resistive layer 22 is in a normally spacedrelationship (i.e., the switch is normally open) relative to the contactconductors 13. The glued or fixed cover is arranged to permit leakage ofair; otherwise, an air passageway must be provided as referred to inother embodiments hereinafter.

Referring to FIG. 3, in an alternative embodiment of the invention, thepressure sensitive resistive layer 42 is disposed immediately on top ofthe contact conductors 13 and a conductor layer 36, such as a very thinlayer of silver, is disposed on the surface of the support member facingthe resistive layer 42 on the contact conductors 13.

Of course other arrangements of the present invention are possible solong as a pressure sensitive semiconductor composition layer ispositioned between the contact conductors 13 and the cover 19 so thatwhen the cover 19 is depressed into a contacting relationship with thecontact conductors 13, the pressure sensitive resistive compositionlayer 22, 42 or 62 (FIGS. 2, 3 or 1) will be in series between a firstcontact conductor and a second contact conductor. By exerting more orless pressure to the resistive composition layer, more or less surfacecontact is made causing increased resistance between the adjacentconductors.

Referring again to FIG. 2 as well as FIG. 3, when the support member 20is depressed, air trapped in the enclosure 24 will be compressed and canbe exhausted through, for example, the junction between the cover 19 andthe insulative spacer 18 or between the insulative spacer 18 and thebase member 12. When the pressure is then removed from the cover 19, theresilient forces of the support member 20 will be insufficient toovercome the partial vacuum thus created in the enclosure 24, causingthe cover 19 to remain in a depressed or closed state. This prevents theswitch 10 from returning to a normally open state. In order to avoidthis vacuum problem, a passageway in the form of an orifice 26 extendingthrough the base member 12 allows air to flow into and out of theenclosure 24 when the cover is released or depressed. Of course it willbe appreciated that any other suitable pressure release mechanism may beincorporated and for example the orifice 26 may be positioned throughthe cover 19 or through the insulative spacer 18. However, in thepreferred embodiment the passageway will be the orifice 26 in the basemember 12.

Referring now to FIG. 4, a conductor pattern which may be used inaccordance with the present invention is illustrated schematically.Specifically, a pressure responsive variable contact resistance analogswitch is shown with the cover removed to illustrate the contactconductor patterns 14 and 16 and their interconnection to a utilizationcircuit 28. Specifically, a first lead 32 is interconnected to one inputof a utilization circuit 28 and terminates in a multiple diameter,opened ring, first conductor pattern 16. A second lead 34 is coupledbetween a second terminal of the utilization circuit 28 and a secondcontact conductor pattern 14 also comprised of a plurality of openedcircular conductors of varying diameters. The circular portions of thefirst and second conductors 16 and 14 respectively are interleavedbetween one another in spaced-apart relationship and are disposed on abase member 12 with the insulative spacer such as an insulative ring 18,disposed around the periphery of the contact conductors 13. Thus, bydepressing the cover 19 an electrical path will be provided through aresistance 31 provided by the semiconductor composition layer betweenthe first conductor 16 and the second conductor 14.

The range of resistance valves which may be inserted between conductors32 and 34 by applying pressure may be increased by increasing thespacing between the interleaved conductors 16 and 14 or by varying thewidth of the conductors.

While particular embodiments of the invention have been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects, and therefore the aim in the appended claims is tocover all such changes and modifications as followed in the true spiritand scope of the invention.

What is claimed is:
 1. A pressure responsive analog transducercomprising:a first contact; a second contact; at least onesemiconducting layer comprising at least a pressure sensitivesemiconductor particulate material, the semiconducting layer disposed innormally spaced relationship with at least one of the first and seconcontacts, the semiconducting layer having a surface positioned innormally non-electrically-conducting relationship with at least one ofthe first and second contacts, the surface having a multiplicity ofmicroprotrusions extending therefrom, each providing a minute contactlocation, at least one of the first contact, second contact andsemiconducting layer being resiliently responsive to an externalpressing force for causing the multiplicity of microprotrusionsextending from the surface of the semiconducting layer and at least oneof the first and second contacts to variably press against one anotherto define a variably electrically resistive junction so that electricityconducts between the first and second contacts through the minutecontact location of the microprotrusions on the surface so that theresistance across the variably electrically resistive junction decreasesin response to an increase in the external pressing force and increasesin response to a decrease in the external pressing force.
 2. The analogtransducer of claim 1 further comprising a cover wherein thesemiconducting layer is disposed on the surface of the cover forproviding a shunt between the first contact and the second contact whenthe pressing force is applied.
 3. The analog transducer of claim 1further comprising:a cover; and a third contact on the surface of thecover in facing relationship to the first and second contacts forselectively forming a shunt between the first and second contacts, thesemiconducting layer being disposed to overlay at least one of thefirst, second and third contacts.
 4. The analog transducer of claims 1,2 or 3 wherein the semiconducting layer further comprises a bindercomposition mixed with the semiconductor particulate material and thesemiconductor particulate material comprises particulated molybdenumdisulfide.
 5. The analog transducer of claim 4 wherein the molybdenumdisulfide particles have a maximum diameter which is less than about 10microns.
 6. The analog transducer of claim 4 wherein the bindercomposition comprises an acrylic resin.
 7. The analog transducer ofclaim 4 wherein the particulated molybdenum disulfide and binder mixtureis applied so that the semiconducting layer has a substantially constantthickness and the exposed surface has a multiplicity of protrudingparticles each comprising a minute contact location.
 8. The analogtransducer of claim 4 wherein the weight ratio of binder to particulatedmolybdenum disulfide is about one-to-one.
 9. The analog transducer ofclaim 5 wherein the weight ratio of binder to particulated molybdenumdisulfide is about one-to-one.